This invention relates to an aqueous release coating composition for pressure sensitive adhesives.
Pressure sensitive adhesives have widespread use as bonding agents for films, foil, and paper for tapes, labels and decals. Films, foil, and paper for tapes, labels and decals will be hereafter referred to as "backing". The adhesive may be coated onto one or both sides of the backing. The side of the backing coated with adhesive will be referred to hereafter as the "adhesive-coated backing side".
In tapes, for example, the adhesive-coated backing is typically wound onto itself to form a roll where the adhesive-coated backing side is in direct contact with the upper surface of the continuous backing layer in the roll which typically does not contain adhesive. The side of the backing not coated with adhesive but coated with a material which functions as a release agent will be referred to hereafter as the "release-coated backing side". In certain cases, both sides of the backing may be coated with adhesive and a release-coat paper containing a separate backing and a release coating may be applied onto one or both sides of these adhesive-coated backing sides. Hereinafter a backing having both sides coated with adhesive will be referred to as "double adhesive coated backing" and the side or sides which contain the release-coat paper will be referred to as the "release-coated paper side." The adhesive must release cleanly from the release-coated backing side or release-coated paper side without causing damage or leaving any adhesive residue on the release-coated backing side or release-coated paper side.
Forming rolls from the adhesive-coated backing imposes an additional requirement on the adhesive. The adhesive must have differential affinity for the two sides of the backing, such that the adhesive preferentially adheres to the coated backing side with no subsequent transfer to the uncoated backing side of the adjacent layer when the backing is unrolled.
There are two methods which result in differential affinity of the adhesive towards the two sides of the backing--the primer coating method and the release coating method. In the primer coating method, an adhesive is selected which does not adhere to the backing and a primer coat is used to make the adhesive bond to the backing. The primer coat is first applied to the backing and then the adhesive is applied onto the primer coat. When the adhesive-coated backing is rolled, the adhesive on the adhesive-coated backing side contacts the backing side which does not contain adhesive but does not adhere to it. Examples of this method are solvent-based natural rubber adhesives coated on unplasticized polyvinyl chloride film and aqueous-based acrylic adhesives coated on oriented polypropylene film where the coated backing side has been "primed" by corona discharge treatment.
In the release coating method, a release coat is applied to the backing side which does not contain adhesive ("release-coated backing side") and an adhesive is applied the opposite backing side ("adhesive-coated backing side"). The release coat has a strong affinity for the backing but a weak affinity for the adhesive. Release coats are typically used with solvent-based natural rubber adhesives and hot-melt block copolymers coated on oriented polypropylene film.
Release coating may alternatively may be applied to a separate backing which is subsequently applied to the adhesive-coated side of the backing (hereinafter referred to as "release-coat paper.")
There are several requirements which a coating must meet to function as a release coating. It must exhibit low affinity toward the adhesive with which it will directly contact when the backing is wound into a roll. This low affinity must be maintained over time and under temperature and humidity fluctuations and extremes. However, the release coating must not reduce the adhesive bonding between the backing and the adhesive to such an extent that the roll "telescopes." The release coating must also strongly bond to the backing side so that it does not delaminate when aggressive adhesives are used. Further, components of the release coating must not migrate into the adhesive, thereby deteriorating the properties of the adhesive.
It is also desirable for safety, health and environmental reasons to use a release coat which is aqueous and does not contain solvents.